1. Field of the Invention
The invention relates to fiber optic sensor systems. More particularly, the invention relates to methods and apparatus for enhancing dynamic range, sensitivity, accuracy, and resolution in FBG fiber optic sensor systems. As used herein, the term xe2x80x9cdynamic rangexe2x80x9d refers to the difference between the lowest value and the highest value a sensor is capable of detecting; the term xe2x80x9csensitivityxe2x80x9d refers to the lowest value a sensor is capable of detecting; the term xe2x80x9caccuracyxe2x80x9d refers to whether the values detected by the sensor are truly representative of what is being sensed; and the term xe2x80x9cresolutionxe2x80x9d refers to how many units of values can be detected over a sensor""s dynamic range.
2. State of the Art
Fiber optic sensor technology has developed concurrently with fiber optic telecommunication technology. The physical aspects of optical fibers which enable them to act as wave guides for light are affected by environmental influences such as temperature, pressure, and strain. These aspects of optical fibers which may be considered a disadvantage to the telecommunications industry are an important advantage to the fiber optic sensor industry.
Optical fibers, whether used in telecommunications or as environmental sensors, generally include a cylindrical core, a concentric cylindrical cladding surrounding the core, and a concentric cylindrical protective jacket or buffer surrounding the cladding. The core is made of transparent glass or plastic having a certain index of refraction. The cladding is also made of transparent glass or plastic, but having a different, smaller, index of refraction. The ability of the optical fiber to act as a bendable waveguide is largely determined by the relative refractive indices of the core and the cladding.
The refractive index of a transparent medium is the ratio of the velocity of light in a vacuum to the velocity of light in the medium. As a beam of light enters a medium, the change in velocity causes the beam to change direction. More specifically, as a beam of light travels from one medium into another medium, the beam changes direction at the interface of the two media. In addition to changing direction at the interface of two media, a portion of the incident beam is reflected at the interface such that the energy of the beam travelling through the second medium is diminished (the sum of the energy of the refracted and reflected beams must equal the energy of the incident beam). The angles of reflection and refraction can be predicted using Snell""s law if the refractive indices of both media are known.
By altering the indices of refraction of two adjacent media, the angle of refraction and the angle of reflection of a beam travelling toward the interface of the two media can be altered such that the intensity of the light entering the second medium approaches zero and substantially all of the light is reflected at the interface. Conversely, for any two transparent media, there is a critical angle of incidence at their interface at or below which substantially all of the incident light will be reflected. This phenomenon, known as total internal reflection, is applied in choosing the refractive indices of the core and the cladding in optical fibers so that light may propagate through the core of the fiber with minimal power loss.
Many other factors affect the propagation of light through the fiber optic core, including the dimensions of the core and the cladding, the wavelength of the light, the magnetic field vectors of the light and electrical field vectors of the light. In addition, many of the physical laws used to determine the ideal propagation of light through a wave guide (optical fiber) assume an xe2x80x9cidealxe2x80x9d wave guide, i.e. a straight wave guide with perfect symmetry and no imperfections. For example, the diameter of the core and the wavelength of the light transmitted through it will determine whether the fiber optic is xe2x80x9csingle modelxe2x80x9d or xe2x80x9cmultimodexe2x80x9d. The terms single mode and multimode refer to the dimensional orientation of rays propagating through the fiber. Single mode fibers have a core with a relatively small diameter (2-12 microns) and support only one spatial mode of propagation. Multimode fibers have a core with a relatively large diameter (25-75 microns) and permit non-axial rays or modes to propagate through the core. The so-called single mode fibers are actually two mode fibers in the sense that there are two different states of optical polarization that can be propagated through the core. In an ideal, straight, imperfection-free fiber with perfect circular symmetry, the propagation velocity of light is independent of the direction of polarization.
A fiber with an elliptical core will have two preferred directions of polarization (along the major axis and along the minor axis). Linearly polarized light injected into the fiber at any other direction of polarization will propagate in two separate modes that travel at slightly different velocities. This type of fiber is said to have a xe2x80x9cmodal birefringencexe2x80x9d. In a real fiber of this type, even ideally polarized light will couple into the other mode due to imperfections in the core-cladding interface, index of refraction fluctuations, and other mechanisms. Static and dynamic changes in polarization may occur along the entire length of the fiber. Over a given distance, the phases of the two modes will pass through an entire cycle of being in phase and out of phase. This distance is known as the xe2x80x9cbeat lengthxe2x80x9d. A long beat length is associated with a small birefringence and a short beat length is associated with a large birefringence. Birefringent optical fibers are also known as xe2x80x9cpolarization preserving fibersxe2x80x9d or xe2x80x9cpolarization maintaining (PM) fibersxe2x80x9d. Birefringence is achieved by providing a core with an elliptical cross section or by providing circular core with a cladding which induces stress on the core. For example, the cladding may be provided with two parallel stress members having longitudinal axes which lie in the same plane as the axis of the core.
As mentioned above, fiber optic sensor employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. The primary advantages of fiber optic sensors include their ability to be light weight, very small, passive, energy efficient, rugged, and immune to electromagnetic interference. In addition, fiber optic sensors have the potential for very high sensitivity, large dynamic range, and wide bandwidth. Further, a certain class of fiber sensors may be distributed or multiplexed along a length of fiber. They may also be embedded into materials.
State of the art fiber optic sensors can be classified as either xe2x80x9cextrinsicxe2x80x9d or xe2x80x9cintrinsicxe2x80x9d. Extrinsic sensors rely on some other device being coupled to the fiber optic in order to translate environmental effects into changes in the properties of the light in the fiber optic. Intrinsic sensors rely only on the properties of the optical fiber in order to measure ambient environmental effects. Known fiber optic sensors include linear position sensors, rotational position sensors, fluid level sensors, temperature sensors, strain gauges, fiber optic gyroscopes, and pressure sensors.
One type of fiber optic pressure sensor takes advantage of the fact that ambient pressure places a strain on the jacket of an optical fiber which strains the cladding, thereby straining the core and changing the birefringence of the fiber. When a force is applied transversely to the fiber, the birefringence of the fiber changes, which changes the beat length and thus the intensity of light viewed by an intensity detector. Another type of fiber optic sensor utilizes intra-core fiber gratings as disclosed in U.S. Pat. No. 5,380,995 to Udd et al., the complete disclosure of which is incorporated by reference herein. Intra-core Bragg gratings are formed in a fiber optic by doping an optical fiber with material such as germania and then exposing the side of the fiber to an interference pattern to produce sinusoidal variations in the refractive index of the core. Two presently known methods of providing the interference pattern are by holographic imaging and by phase mask grating. Holographic imaging utilizes two short wavelength (usually 240 nm) laser beams which are imaged through the side of a fiber core to form the interference pattern. The bright fringes of the interference pattern cause the index of refraction of the core to be xe2x80x9cmodulatedxe2x80x9d resulting in the formation of a fiber grating. Similar results are obtained using short pulses of laser light, writing fiber gratings line by line through the use of phase masks. By adjusting the fringe spacing of the interference pattern, the periodic index of refraction can be varied as desired. Another method of writing the grating on the fiber is to focus a laser through the side of the fiber and write the grating one line at a time.
When a fiber optic is provided with a grating and subjected to transverse strain, two spectral peaks are produced (one for each polarization axis) and the peak to peak separation is proportional to the transverse strain. Spectral demodulation systems such as tunable Fabry-Perot filters, acousto-optical filters, or optical spectrum analyzers coupled to the fiber detect the two spectral outputs. The spectral outputs are analyzed and the transverse strain is determined by measuring the peak to peak separation. Depending on how the fiber optic is deployed, the transverse strain may be related to temperature, pressure, or another environmental measure.
There are two shortcomings of this type of sensor system. First, dual peaks are only discernable in ordinary single mode fiber when there is considerable transverse strain, e.g. at very high pressure. Various structures are disclosed for mechanically influencing the fiber such that isotropic forces are converted to anisotropic forces to produce birefringence and to magnify the effect of transverse strain on birefringence. Exemplary structures are disclosed in co-owned U.S. Pat. No. 5,841,131. Nevertheless, mechanical structures can only do so much to enhance the sensitivity of fiber optic sensors. One of the disadvantages of the mechanical structure solution is that it cannot be retro-fitted to existing sensors which are embedded in materials.
The other shortcoming of this type of sensor involves characteristics of the spectral demodulation system. The inventors have discovered that, depending on the polarization of the light detected, detection errors of about xc2x120 picometers occur when using at least some types of Fabry-Perot filters. The polarization state of the light viewed by the demodulation system is changed by lifting, rotating, or twisting any part of the fiber between the Bragg grating and the demodulation system. Therefore, in practical applications, the polarization state of the light viewed by the demodulation system cannot be accurately predicted.
It is therefore an object of the invention to provide methods and apparatus for enhancing dynamic range and resolution in a fiber optic sensor system utilizing a fiber Bragg grating (FBG).
It is also an object of the invention to provide methods and apparatus for increasing the sensitivity of FBG fiber optic sensor systems.
It is another object of the invention to provide methods and apparatus for minimizing errors in measurement by spectral demodulation systems in FBG fiber optic sensor systems.
In accord with these objects which will be discussed in detail below, the methods of the present invention include manipulating the polarization characteristics of the light entering a fiber optic sensor and/or manipulating the polarization characteristics of the light exiting the sensor before it enters the light detection system. It has been discovered by the inventors that by polarizing the light entering or leaving the system to certain orientations, dual peaks can be discerned with relatively low transverse strain. Further, the inventors have discovered that the errors introduced by a spectral demodulation system can be substantially reduced when a depolarizing xe2x80x9cscramblerxe2x80x9d is deployed between the fiber and the spectral demodulation system. An apparatus according to the invention includes a fiber optic core having one or more gratings written onto it, a light source (such as an LED, a laser, a wavelength tunable light source, or a laser diode) and a detection system for detecting light transmitted through the grating or reflected by the grating. The light source and the detection system may be coupled to opposite ends of the fiber optic (transmission mode) or may be coupled to the same end of the fiber optic through the use of a beam splitter and terminators (reflection mode). The methods of the invention may be implemented in several different apparatus depending on what kind of light source is used, what kind of detection system is used, etc.
According to a first embodiment, a polarizer and controller is placed in front of a broad band light source so that light entering the fiber optic sensor is polarized. An optional depolarizing xe2x80x9cscramblerxe2x80x9d is also placed in front of the detection system which includes a spectral demodulator and photo detector. As the polarization of the light entering the sensor is adjusted to the correct orientation, a first peak is detected. Further adjustment of the polarization (xc2x190xc2x0) allows a second peak to be detected. An alternative first embodiment utilizes a polarized light source and polarization rotator in lieu of the unpolarized broad band light source, the polarizer and controller. The detection system and the light source in all embodiments may be located either at opposite ends of the fiber optic, or via use of a beam splitter, at the same end of the fiber optic.
A second embodiment of the invention is similar to the first embodiment except that the polarizer and controller is placed between the sensor and the scrambler.
A third embodiment of the invention is similar to the second embodiment except that two separate detection systems, two scramblers, and a polarizing beam splitter are provided. Also, instead of a polarizer and a controller, a polarization rotator is provided between the sensor and the polarizing beam splitter. Light from the polarization rotator enters the beam splitter where two orthogonally polarized beams exit, one toward each detection system. As each detection system includes a spectral demodulator, a scrambler is provided between the beam splitter and each detector. When the angle of the polarized light exiting the sensor is adjusted to the correct orientation using the polarization rotator, the beam splitter provides two beams each of which represent one of the two peaks. The two peaks are therefor detected simultaneously, one by each of the two detection systems.
According to a fourth embodiment, a polarizer and controller is placed in front of a tunable light source and no special equipment is placed between the sensor and the detection system which does not include a spectral demodulator. The wavelength of the light source is tuned through the wavelength of the grating. As the polarization of the light entering the sensor is adjusted to the correct orientation, a first peak is detected, and further adjustment of the polarization (xc2x190xc2x0) and the wavelength of the light source allows a second peak to be detected. If the system does not contain any components which are adversely affected by polarized light a scrambler may not be needed.
A fifth embodiment is similar to the fourth embodiment except that the polarizer and controller is placed between the sensor and the detection system rather than the light source. The wavelength of the light source is tuned through the wavelength of the grating. As the polarization of the light entering the sensor is adjusted to the correct orientation, a first peak is detected, and further adjustment of the polarization (xc2x190xc2x0) and the wavelength of the light source allows a second peak to be detected. If the system does not contain any components which are adversely affected by polarized light a scrambler may not be needed.
A sixth embodiment of the invention is similar to the fifth embodiment except that two detection systems are provided and a polarizing beam splitter is used to couple the two detection systems to the polarization controller.
The invention can be used in many different types of fiber optic sensors including sensors which include multiple gratings and light sources with multiple frequencies. Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.